'Sister Size’ For Bras: What Are They and Do They Work?

a pile of pretty bras

The knocker locker. The chest choker. The over-the-shoulder boulder holder. Ah yes, the good ol’ fashioned bra.

This supportive undergarment is somewhat of a necessary evil. (Although, there’s actually no scientific or medical reason you need to wear a bra, unless you’re working out or you find it eases back pain.) Regardless, shopping for this particular piece of lingerie is about as fun as its many nicknames suggest. But that doesn’t mean we don’t have a few tricks up our cups sleeves to make the process a bit easier. For instance, did you know there are actually three potential sizes you could be wearing? Your true size and your two sister sizes. Here, let’s break it down.

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What is a sister size?

Sister sizes are bras with the same cup volume. So, in theory your sister size could be swapped for your true size and fit just as perfectly, while the band would fit a bit tighter or a bit looser than usual. 

How do I find my sister size?

To get to your lower sister size, go down a band size and up a cup size. If you typically wear a 34C, your lower sister size would be a 32D. To find your higher size, do just the opposite; go up one band size (in this case to a 36) and down one cup size (leaving you with a 36B). See? Easy as can be!

But how and Why does that even work?

To really understand sister sizing we have to look at how to determine your real size, once and for all. That requires doing a bit of measuring and math.

To find your band, take a tape measure and determine the circumference of your rib cage, aka just under your breasts where the band of your bra sits. Round down to the nearest even number (unless you’re within a quarter inch of the next highest even number, then you can go up) and bam, you’ve got the first half of your size.

To determine your cup, measure the circumference of the fullest part of your chest (typically about mid-boob). Round to the nearest whole number, then subtract your band size. Your cup size corresponds to the difference between those two numbers, so one inch would be an A cup, two a B cup, three a C cup and so on.

Because cup size is directly related to band size—and not a strict numerical measurement on its own—not all cups of a certain letter are equal. In other words, not all Ds are the same (for example, 32D boobs are smaller than 40D boobs). This also means that different band-cup size combos can actually provide the same cup volume.

Gotcha. So, do sister sizes really fit as well as my exact size?

Straight answer:No, they probably won’t. However, if you prefer a tighter or a looser band, it can be helpful to know that you just have to switch up your cup size. And if you’ve really got your heart set on some foxy lingerie that’s out of stock in your go-to measurements you can probably get away with buying it in your sister size. This system can also help you streamline the process of finding your best fitting bra, especially if you’re shopping with a new brand.

Sister sizes also might lead you to discovering you’ve actually been wearing the wrong size bra all along. A lot of women wear their bras too loose, so next time you head out to pick up some new brassieres, why not give your lower sister size a try? You might find it fits just as well with an extra touch of support you never realized was missing. 

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Abby Hepworth is an RRCA-certified running coach who has worked in fashion for over 10 years. Want to know what shoes are in this season? She's got you. Need recommendations on...